System and method for compensation of non-uniformities in light emitting device displays

ABSTRACT

A system and method for operating a display at a constant luminance even as some of the pixels in the display are degraded over time. Each pixel in the display is configured to emit light when a voltage is supplied to the pixel&#39;s driving circuit, which causes a current to flow through a light emitting element. Degraded pixels are compensated by supplying their respective driving circuits with greater voltages. The display data is scaled by a compression factor less than one to reserve some voltage levels for compensating degraded pixels. As pixels become more degraded, and require additional compensation, the compression factor is decreased to reserve additional voltage levels for use in compensation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of prior application Ser. No.11/402,624, filed Apr. 12, 2006, which claims priority to CanadianPatent No. 2,504,571, filed Apr. 12, 2005, each of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to display technologies, more specificallya method and system for compensating for non-uniformities of elements inlight emitting device displays.

BACKGROUND

Active-matrix organic light-emitting diode (AMOLED) displays are wellknown art. Amorphous silicon is, for example, a promising material forAMOLED displays, due to its low cost and vast installed infrastructurefrom thin film transistor liquid crystal display (TFTLCD) fabrication.

All AMOLED displays, regardless of backplane technology used, exhibitdifferences in luminance on a pixel to pixel basis, primarily as aresult of process or construction inequalities, or from aging caused byoperational use over time. Luminance non-uniformities in a display mayalso arise from natural differences in chemistry and performance fromthe OLED materials themselves. These non-uniformities must be managed bythe AMOLED display electronics in order for the display device to attaincommercially acceptable levels of performance for mass-market use.

FIG. 1 illustrates an operational flow of a conventional AMOLED display10. Referring to FIG. 1, a video source 12 contains luminance data foreach pixel and sends the luminance data in the form of digital data 14to a digital data processor 16. The digital data processor 16 mayperform some data manipulation functions, such as scaling the resolutionor changing the color of the display. The digital data processor 16sends digital data 18 to a data driver integrated circuit (IC) 20. Thedata driver IC 20 converts that digital data 18 into an analog voltageor current 22, which is sent to thin film transistors (TFTs) 26 in apixel circuit 24. The TFTs 26 convert that voltage or current 22 intoanother current 28 which flows through an organic light-emitting diode(OLED) 30. The OLED 30 converts the current 28 into visible light 36.The OLED 30 has an OLED voltage 32, which is the voltage drop across theOLED. The OLED 30 also has an efficiency 34, which is a ratio of theamount of light emitted to the current through the OLED.

The digital data 14, analog voltage/current 22, current 28, and visiblelight 36 all contain the exact same information (i.e. luminance data).They are simply different formats of the initial luminance data thatcame from the video source 12. The desired operation of the system isfor a given value of luminance data from the video source 12 to alwaysresult in the same value of the visible light 36.

However, there are several degradation factors which may cause errors onthe visible light 36. With continued usage, the TFTs will output lowercurrent 28 for the same input from the data driver IC 20. With continuedusage, the OLED 30 will consume greater voltage 32 for the same inputcurrent. Because the TFT 26 is not a perfect current source, this willactually reduce the input current 28 slightly. With continued usage, theOLED 30 will lose efficiency 34, and emit less visible light for thesame current.

Due to these degradation factors, the visible light output 36 will beless over time, even with the same luminance data being sent from thevideo source 12. Depending on the usage of the display, different pixelsmay have different amounts of degradation.

Therefore, there will be an ever-increasing error between the requiredbrightness of some pixels as specified by the luminance data in thevideo source 12, and the actual brightness of the pixels. The result isthat the decreased image will not show properly on the display.

One way to compensate for these problems is to use a feedback loop. FIG.2 illustrates an operational flow of a conventional AMOLED display 40that includes the feedback loop. Referring to FIG. 2, a light detector42 is employed to directly measure the visible light 36. The visiblelight 36 is converted into a measured signal 44 by the light detector42. A signal converter 46 converts the measured visible light signal 44into a feedback signal 48. The signal converter 46 may be ananalog-to-digital converter, a digital-to-analog converter, amicrocontroller, a transistor, or another circuit or device. Thefeedback signal 48 is used to modify the luminance data at some pointalong its path, such as an existing component (e.g. 12, 16, 20, 26, 30),a signal line between components (e.g. 14, 18, 22, 28, 36), orcombinations thereof.

Some modifications to existing components, and/or additional circuitsmay be required to allow the luminance data to be modified based on thefeedback signal 48 from the signal converter 46. If the visible light 36is lower than the desired luminance from video source 12, the luminancesignal may be increased to compensate for the degradation of the TFT 26or the OLED 30. This results in that the visible light 36 will beconstant regardless of the degradation. This compensation scheme isoften known as Optical Feedback (OFB). However, in the system of FIG. 2,the light detector 42 must be integrated onto a display, usually withineach pixel and coupled to the pixel circuitry. Not considering theinevitable issues of yield when integrating a light detector into eachpixel, it is desirable to have a light detector which does not degradeitself, however such light detectors are costly to implement, and notcompatible with currently installed TFT-LCD fabrication infrastructure.

Therefore, there is a need to provide a method and system which cancompensate for non-uniformities in displays without measuring a lightsignal.

AMOLED displays are conventionally operated according to digital datafrom a video source. The OLEDs within the display can be programmed toemit light with luminance according to a programming voltage or aprogramming current. The programming current or programming voltage areconventionally set by a display driver that takes digital data as inputand has an analog output for sending the programming current orprogramming voltage to pixel circuits. The pixel circuits are configuredto drive current through OLEDs based on the programming current orprogramming voltage.

SUMMARY

It is an object of the invention to provide a method and system thatobviates or mitigates at least one of the disadvantages of existingsystems.

In accordance with an aspect of the present invention there is provideda system for compensating non-uniformities in a light emitting devicedisplay which includes a plurality of pixels and a source for providingpixel data to each pixel circuit. The system includes: a module formodifying the pixel data applied to one or more than one pixel circuit,an estimating module for estimating a degradation of a first pixelcircuit based on measurement data read from a part of the first pixelcircuit, and a compensating module for correcting the pixel data appliedto the first or a second pixel circuit based on the estimation of thedegradation of the first pixel circuit.

In accordance with a further aspect of the present invention there isprovided a method of compensating non-uniformities in a light emittingdevice display having a plurality of pixels, including the steps of:estimating a degradation of the first pixel circuit based on measurementdata read from a part of the first pixel circuit, and correcting pixeldata applied to the first or a second pixel circuit based on theestimation of the degradation of the first pixel circuit.

The present disclosure provides a method of maintaining uniformluminosity of an AMOLED display. The AMOLED display includes an array ofpixels having light emitting devices. The light emitting devices areconfigured to emit light according to digital input from a video source.The video source includes digital data corresponding to a desiredluminance of each pixel in the AMOLED display. Over time, aspects withinthe light emitting devices and their associated driving circuits degradeand require compensation to continue to emit light with the sameluminance for a given digital input.

Degradation of the pixels in the light emitting display are compensatedby incrementing the digital inputs of the pixels according to a measuredor estimated degradation of the pixels. To allow for compensation tooccur, the digital input is compressed to a range of values less than anavailable range. Compressing the digital input is carried out accordingto a compression factor, which is a number less than one. In animplementation of the present disclosure, the digital inputs aremultiplied by the compression factor, which compresses the digital inputto a range less than the available range. The remaining portion of thedigital range can be used to provide compensation to degraded pixelsbased on measured or estimated degradation of the pixels. The presentdisclosure provides methods for setting and adjusting the compressionfactor to statically or dynamically adjust the compression factor andprovide compensation to the display by incrementing the digital signalsbefore the signals are sent to the driving circuits.

The foregoing and additional aspects and embodiments of the presentinvention will be apparent to those of ordinary skill in the art in viewof the detailed description of various embodiments and/or aspects, whichis made with reference to the drawings, a brief description of which isprovided next.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings.

FIG. 1 illustrates a conventional AMOLED system.

FIG. 2 illustrates a conventional AMOLED system that includes a lightdetector and a feedback scheme that uses the signal from the lightdetector.

FIG. 3 illustrates a light emitting display system to which acompensation scheme in accordance with an embodiment of the presentinvention is applied.

FIG. 4 illustrates an example of the light emitting display system ofFIG. 3.

FIG. 5 illustrates an example of a pixel circuit of FIG. 5.

FIG. 6 illustrates a further example of the light emitting displaysystem of FIG. 3.

FIG. 7 illustrates an example of a pixel circuit of FIG. 6.

FIG. 8 illustrates an example of modules for the compensation schemeapplied to the system of FIG. 4.

FIG. 9 illustrates an example of a lookup table and a compensationalgorithm module of FIG. 7.

FIG. 10 illustrates an example of inputs to a TFT-to-pixel circuitconversion algorithm module.

FIG. 11A illustrates an experimental result of a video source outputtingequal luminance data for each pixel for a usage time of zero hours.

FIG. 11B illustrates an experimental result of a video source outputtingmaximum luminance data to some pixels and zero luminance data to otherpixels for a usage of time of 1000 hours.

FIG. 11C illustrates an experimental result of a video source outputtingequal luminance data for each pixel after some pixels received maximumluminance data and others pixels received zero luminance data for ausage time of 1000 hours when no compensation algorithm is applied.

FIG. 11D illustrates an experimental result of a video source outputtingequal luminance data for each pixel after some pixels received maximumluminance data and others pixels received zero luminance data for ausage time of 1000 hours when a constant brightness compensationalgorithm is applied.

FIG. 11E illustrates an experimental result of a video source outputtingequal luminance data for each pixel after some pixels received maximumluminance data and others pixels received zero luminance data for ausage time of 1000 hours when a decreasing brightness compensationalgorithm is applied.

FIG. 12 illustrates an example of a grayscale compression algorithm.

FIG. 13 is a data flow chart showing the compression and compensation ofluminosity input data used to drive an AMOLED display.

FIG. 14 is a flowchart illustrating a method for selecting thecompression factor according to display requirements and the design ofthe pixel circuit.

FIG. 15 is a flowchart illustrating a method for selecting thecompression factor according to a pre-determined headroom adjustmentprofile.

FIG. 16 is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a threshold over a previous compensation.

FIG. 17 is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a previously measured maximum.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. Rather, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Embodiments of the present invention are described using an AMOLEDdisplay which includes a pixel circuit having TFTs and an OLED. However,the transistors in the pixel circuit may be fabricated using amorphoussilicon, nano/micro crystalline silicon, poly silicon, organicsemiconductors technologies (e.g. organic TFT), NMOS technology, CMOStechnology (e.g. MOSFET), or combinations thereof. The transistors maybe a p-type transistor or n-type transistor. The pixel circuit mayinclude a light emitting device other than OLED. In the descriptionbelow, “pixel” and “pixel circuit” may be used interchangeably.

FIG. 3 illustrates the operation of a light emitting display system 100to which a compensation scheme in accordance with an embodiment of thepresent invention is applied. A video source 102 contains luminance datafor each pixel and sends the luminance data in the form of digital data104 to a digital data processor 106. The digital data processor 106 mayperform some data manipulation functions, such as scaling the resolutionor changing the color of the display. The digital data processor 106sends digital data 108 to a data driver IC 110. The data driver IC 110converts that digital data 108 into an analog voltage or current 112.The analog voltage or current 112 is applied to a pixel circuit 114. Thepixel circuit 114 includes TFTs and an OLED. The pixel circuit 114outputs a visible light 126 based on the analog voltage or current 112.

In FIG. 3, one pixel circuit is shown as an example. However, the lightemitting display system 100 includes a plurality of pixel circuits. Thevideo source 102 may be similar to the video source 12 of FIGS. 1 and 2.The data driver IC 110 may be similar to the data driver 110 may besimilar to the data driver IC 20 of FIGS. 1 and 2.

A compensation functions module 130 is provided to the display. Thecompensation functions module 130 includes a module 134 for implementingan algorithm (referred to as TFT-to-pixel circuit conversion algorithm)on measurement 132 from the pixel circuit 114 (referred to asdegradation data, measured degradation data, measured TFT degradationdata, or measured TFT and OLED degradation data), and outputs calculatedpixel circuit degradation data 136. It is noted that in the descriptionbelow, “TFT-to-pixel circuit conversion algorithm module” and“TFT-to-pixel circuit conversion algorithm” may be used interchangeably.

The degradation data 132 is electrical data which represents how much apart of the pixel circuit 114 has been degraded. The data measured fromthe pixel circuit 114 may represent, for example, one or morecharacteristics of a part of the pixel circuit 114.

The degradation data 132 is measured from, for example, one or morethin-film-transistors (TFTs), an organic light emitting diode (OLED)device, or a combination thereof. It is noted that the transistors ofthe pixel circuit 114 are not limited to TFTs, and the light emittingdevice of the pixel circuit 114 is not limited to an OLED. The measureddegradation data 132 may be digital or analog data. The system 100provides compensation data based on measurement from a part of the pixelcircuit (e.g. TFT) to compensate for non-uniformities in the display.The non-uniformities may include brightness non-uniformity, colornon-uniformity, or a combination thereof. Factors for causing suchnon-uniformities may include, but are not limited to, process orconstruction inequalities in the display, aging of pixels, etc.

The degradation data 132 may be measured at a regular timing or adynamically regulated timing. The calculated pixel circuit degradationdata 136 may be compensation data to correct non-uniformities in thedisplay. The calculated pixel circuit degradation data 136 may includeany parameters to produce the compensation data. The compensation datamay be used at a regular timing (e.g. each frame, regular interval,etc.) or dynamically regulated timing. The measured data, compensationdata, or a combination thereof may be stored in a memory (e.g. 142 ofFIG. 8).

The TFT-to-pixel circuit conversion algorithm module 134 or thecombination of the TFT-to-pixel circuit conversion algorithm module 134and the digital data processor 106 estimates the degradation of theentire pixel circuit based on the measured degradation data 132. Basedon this estimation, the entire degradation of the pixel circuit 114 iscompensated by adjusting, at the digital data processor 106, theluminance data (digital data 104) applied to a certain pixel circuit(s).

The system 100 may modify or adjust luminance data 104 applied to adegraded pixel circuit or non-degraded pixel circuit. For example, if aconstant value of visible light 126 is desired, the digital dataprocessor 106 increases the luminance data for a pixel that is highlydegraded, thereby compensating for the degradation.

In FIG. 3, the TFT-to-pixel circuit conversion algorithm module 134 isprovided separately from the digital data processor 106. However, theTFT-to-pixel circuit conversion algorithm module 134 may be integratedinto the digital data processor 106.

FIG. 4 illustrates an example of the system 100 of FIG. 3. The pixelcircuit 114 of FIG. 4 includes TFTs 116 and OLED 120. The analog voltageor current 112 is provided to the TFTs 116. The TFTs 116 convert thatvoltage or current 112 into another current 118 which flows through theOLED 120. The OLED 120 converts the current 118 into the visible light126. The OLED 120 has an OLED voltage 122, which is the voltage dropacross the OLED. The OLED 120 also has an efficiency 134, which is aratio of the amount of light emitted to the current through the OLED120.

The system 100 of FIG. 4 measures the degradation of the TFTs only. Thedegradation of the TFTs 116 and the OLED 120 are usage-dependent, andthe TFTs 116 and the OLED 120 are always linked in the pixel circuit114. Whenever the TFT 116 is stressed, the OLED 120 is also stressed.Therefore, there is a predictable relationship between the degradationof the TFTs 116, and the degradation of the pixel circuit 114 as awhole. The TFT-to-pixel circuit conversion algorithm module 134 or thecombination of the TFT-to-pixel circuit conversion algorithm module 134and the digital data processor 106 estimates the degradation of theentire pixel circuit based on the TFT degradation only. An embodiment ofthe present invention may also be applied to systems that monitor bothTFT and OLED degradation independently.

The pixel circuit 114 has a component that can be measured. Themeasurement obtained from the pixel circuit 114 is in some way relatedto the pixel circuit's degradation.

FIG. 5 illustrates an example of the pixel circuit 114 of FIG. 4. Thepixel circuit 114 of FIG. 5 is a 4-T pixel circuit. The pixel circuit114A includes a switching circuit having TFTs 150 and 152, a referenceTFT 154, a dive TFT 156, a capacitor 158, and an OLED 160.

The gate of the switch TFT 150 and the gate of the feedback TFT 152 areconnected to a select line Vsel. The first terminal of the switch TFT154 and the first terminal of the feedback TFT 152 are connected to adata line Idata. The second terminal of the switch TFT 150 is connectedto the gate of the reference TFT 154 and the gate of the drive TFT 156.The second terminal of the feedback TFT 152 is connected to the firstterminal of the reference TFT 154. The capacitor 158 is connectedbetween the gate of the drive TFT 156 and ground. The OLED 160 isconnected between voltage supply Vdd and the drive TFT 156. The OLED 160may also be connected between drive TFT 156 and ground in other systems(i.e. drain-connected format).

When programming the pixel circuit 114A, Vsel is high and a voltage orcurrent is applied to the data line Idata. The data Idata initiallyflows through the TFT 150 and charges the capacitor 158. As thecapacitor voltage rises, the TFT 154 begins to turn on and Idata startsto flow through the TFTs 152 and 154 to ground. The capacitor voltagestabilizes at the point when all of Idata flows through the TFTs 152 and154. The current flowing through the TFT 154 is mirrored in the driveTFT 156.

In the pixel circuit 114A, by setting Vsel to high and putting a voltageon Idata, the current flowing into the Idata node can be measured.Alternately, by setting Vsel to high and putting a current on Idata, thevoltage at the Idata node can be measured. As the TFTs degrade, themeasured voltage (or current) will change, allowing a measure of thedegradation to be recorded. In this pixel circuit, the analogvoltage/current 112 shown in FIG. 4 is connected to the Idata node. Themeasurement of the voltage or current can occur anywhere along theconnection between the data diver IC 110 and the TFTs 116.

In FIG. 4, the TFT-to-pixel circuit conversion algorithm is applied tothe measurement 132 from the TFTs 116. However, current/voltageinformation read from various places other than TFTs 116 may be usable.For example, the OLED voltage 122 may be included with the measured TFTdegradation data 132.

FIG. 6 illustrates a further example of the system 100 of FIG. 3. Thesystem 100 of FIG. 6 measures the OLED voltage 122. Thus, the measureddata 132 is related to the TFT 116 and OLED 120 degradation (“measuredTFT and OLED voltage degradation data 132A” in FIG. 6). The compensationfunctions module 130 of FIG. 6 implements the TFT-to-pixel circuitconversion algorithm 134 on the signal related to both the TFTdegradation and OLED degradation. The TFT-to-pixel circuit conversionalgorithm module 134 or the combination of the TFT-to-pixel circuitconversion algorithm module 134 and the digital data processor 106estimates the degradation of the entire pixel circuit based on the TFTdegradation and the OLED degradation. The TFT degradation and OLEDdegradation may be measured separately and independently.

FIG. 7 illustrates an example of the pixel circuit 114 of FIG. 6. Thepixel circuit 114B of FIG. 7 is a 4-T pixel circuit. The pixel circuit114B includes a switching circuit having TFTs 170 and 172, a referenceTFT 174, a drive TFT 176, a capacitor 178, and an OLED 180.

The gate of the switch TFT 170 and the gate of the switch TFT 172 areconnected to a select line Vsel. The first terminal of the switch TFT172 is connected to a data line Idata while the first terminal of theswitch TFT 170 is connected to the second terminal of the switch TFT 172which is connected to the gate of the reference TFT 174 and the gate ofthe dive TFT 176. The second terminal of the switch TFT 170 is connectedto the first terminal of the reference TFT 174. The capacitor 178 isconnected between the gate of the dive TFT 176 and ground. The firstterminal of the dive TFT 176 is connected to voltage supply Vdd. Thesecond terminal of the reference TFT 174 and the second terminal of thedrive TFT 176 are connected to the OLED 180.

When programming the pixel circuit 114B, Vsel is high and a voltage orcurrent is applied to the data line Idata. The data Idata initiallyflows through the TFT 172 and charges the capacitor 178. As thecapacitor voltage rises, the TFT 174 begins to turn on and Idata startsto flow through the TFTs 170 and 174 and OLED 180 to ground. Thecapacitor voltage stabilizes at the point when all of Idata flowsthrough the TFTs 152 and 154. The current flowing through the TFT 154 ismirrored in the drive TFT 156. In the pixel circuit 114A, by settingVsel to high and putting a voltage on Idata, the current flowing intothe Idata node can be measured. Alternately, by setting Vsel to high andputting a current on Idata, the voltage at the Idata node can bemeasured. As the TFTs degrade, the measured voltage (or current) willchange, allowing a measure of the degradation to be recorded. It isnoted that unlike the pixel circuit 114A of FIG. 5, the current nowflows through the OLED 180. Therefore the measurement made at the Idatanode is now partially related to the OLED voltage, which will degradeover time. In the pixel circuit 114B, the analog voltage/current 112shown in FIG. 6 is connected to the Idata node. The measurement of thevoltage or current can occur anywhere along the connection between thedata driver IC 110 and the TFTs 116.

Referring to FIGS. 3, 4, and 6, the pixel circuit 114 may allow thecurrent out of the TFTs 116 to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may allow somepart of the OLED efficiency to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may also allowa node to be charged, and the measurement may be the time it takes forthis node to discharge. The pixel circuit 114 may allow any parts of itto be electrically measured. Also, the discharge/charge level during agiven time can be used for aging detection.

Referring to FIG. 8, an example of modules for the compensation schemeapplied to the system of FIG. 4 is described. The compensation functionsmodule 130 of FIG. 8 includes an analog/digital (A/D) converter 140. TheA/D converter 140 converts the measured TFT degradation data 132 intodigital measured TFT voltage/current 112 shown in FIG. 4 is connected tothe Idata node. The measurement of the voltage or current can occuranywhere along the connection between the data driver IC 110 and theTFTs 116.

In FIG. 4, the TFT-to-pixel circuit conversion algorithm is applied tothe measurement 132 from the TFTs 116. However, current/voltageinformation read from various places other than TFTs 116 may be usable.For example, the OLED voltage 122 may be included with the measured TFTdegradation data 132.

FIG. 6 illustrates a further example of the system 100 of FIG. 3. Thesystem 100 of the FIG. 6 measured the OLED voltage 122. Thus, themeasured data 132 is related to the TFT 116 and OLED 120 degradation(“measured TFT and OLED voltage degradation data 132A” in FIG. 6). Thecompensation functions module 130 of FIG. 6 implements the TFT-to-pixelcircuit conversion algorithm 134 on the signal related to both the TFTdegradation and OLED degradation. The TFT-to-pixel circuit conversionalgorithm module 134 or the combination of the TFT-to-pixel circuitconversion algorithm module 134 and the digital data processor 106estimates the degradation of the entire pixel circuit based on the TFTdegradation and the OLED degradation. The TFT degradation and OLEDdegradation may be measured separately and independently.

FIG. 7 illustrates an example of the pixel circuit 114 of FIG. 6. Thepixel circuit 114B of FIG. 7 is a 4-T pixel circuit. The pixel circuit114B includes a switching circuit having TFTs 170 and 172, a referenceTFT 174, a drive TFT 176, a capacitor 178, and an OLED 180.

The gate of the switch TFT 170 and the gate of the switch TFT 172 areconnected to a select line Vsel. The first terminal of the switch TFT172 is connected to a data line Idata while the first terminal of theswitch TFT 170 is connected to the second terminal of the switch TFT172, which is connected to the gate of the reference TFT 174 and thegate of the drive TFT 176. The second terminal of the switch TFT 170 isconnected to the first terminal of the reference TFT 174. The capacitor178 is connected between the gate of the drive TFT 176 and ground. Thefirst terminal of the drive TFT 176 is connected to voltage supply Vdd.The second terminal of the reference TFT 174 and the second terminal ofthe drive TFT 176 are connected to the OLED 180.

When programming the pixel circuit 114B, Vsel is high and a voltage orcurrent is applied to the data line Idata. The data Idata initiallyflows through the TFT 172 and charges the capacitor 178. As thecapacitor voltage rises, the TFT 174 begins to turn on and Idata startsto flow through the TFTs 170 and 174 and OLED 180 to ground. Thecapacitor voltage stabilizes at the point when all of Idata flowsthrough the TFTs 152 and 154. The current flowing through the TFT 154 ismirrored in the drive TFT 156. In the pixel circuit 114A, by settingVsel to high and putting a voltage on Idata, the current flowing intothe Idata node can be measured. Alternately, by setting Vsel to high andputting a current on Idata, the voltage at the Idata node can bemeasured. As the TFTs degrade, the measured voltage (or current) willchange, allowing a measure of the degradation to be recorded. It isnoted that unlike the pixel circuit 114A of FIG. 5, the current nowflows through the OLED 180. Therefore the measurement made at the Idatanode is now partially related to the OLED voltage, which will degradeover time. In the pixel circuit 114B, the analog voltage/current 112shown in FIG. 6 is connected to the Idata node. The measurement of thevoltage or current can occur anywhere along the connection between thedata driver IC 110 and the TFTs 116.

Referring to FIGS. 3, 4, and 6, the pixel circuit 114 may allow thecurrent out of the TFTs 116 to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may allow somepart of the OLED efficiency to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may also allowa node to be charged, and the measurement may be the time it takes forthis node to discharge. The pixel circuit 114 may allow any parts of itto be electrically measured. Also, the discharge/charge level during agiven time can be used for aging detection.

Referring to FIG. 8, an example of modules for the compensation schemeapplied to the system of FIG. 4 is described. The compensation functionsmodule 130 of FIG. 8 includes an analog/digital (A/D) converter 140. TheA/D converter 140 converts the measured TFT degradation data 132 intodigital measured TFT degradation data 132B. The digital measured TFTdegradation data 132B is converted into the calculated pixel circuitdegradation data 136 at the TFT-to-pixel circuit conversion algorithmmodule 134. The calculated pixel circuit degradation data 136 is storedin a lookup table 142. Since measuring TFT degradation data from somepixel circuits may take a long time, the calculated pixel circuitdegradation data 136 is stored in the lookup table 142 for use.

In FIG. 8, the TFT-to-pixel circuit conversion algorithm 134 is adigital algorithm. The digital TFT-to-pixel circuit conversion algorithm134 may be implemented, for example, on a microprocessor, an FPGA, aDSP, or another device, but not limited to these examples. The lookuptable 142 may be implemented using memory, such as SRAM or DRAM. Thismemory may be in another device, such as a microprocessor or FPGA, ormay be an independent device.

The calculated pixel circuit degradation data 136 stored in the lookuptable 142 is always available for the digital data processor 106. Thus,the TFT degradation data 132 for each pixel does not have to be measuredevery time the digital data processor 106 needs to use the data. Thedegradation data 132 may be measured infrequently (for example, onceevery 20 hours, or less). Using a dynamic time allocation for thedegradation measurement is another case, more frequent extraction at thebeginning and less frequent extraction after the aging gets saturated.

The digital data processor 106 may include a compensation module 144 fortaking input luminance data for the pixel circuit 114 from the videosource 102, and modifying it based on degradation data for that pixelcircuit or other pixel circuit. In FIG. 8, the module 144 modifiesluminance data using information from the lookup table 142.

It is noted that the configuration of FIG. 8 is applicable to the systemof FIGS. 3 and 6. It is noted that the lookup table 142 is providedseparately from the compensating functions module 130, however, it maybe in the compensating functions module 130. It is noted that the lookuptable 142 is provided separately from the digital data processor 106,however, it may be in the digital data processor 106.

One example of the lookup table 142 and the module 144 of the digitaldata processor 106 is illustrated in FIG. 9. Referring to FIG. 9, theoutput of the TFT-to-pixel circuit conversion algorithm module 134 is aninteger value. This integer is stored in a lookup table 142A(corresponding to 142 of FIG. 8). Its location in the lookup table 142Ais related to the pixel's location on the AMOLED display. Its value is anumber, and is added to the digital luminance data 104 to compensate forthe degradation.

For example, digital luminance data may be represented to use 8-bits(256 values) for the brightness of a pixel. A value of 246 may representmaximum luminance for the pixel. A value of 128 may representapproximately 50% luminance. The value in the lookup table 142A may bethe number that is added to the luminance data 104 to compensate for thedegradation. Therefore, the compensation module (144 of FIG. 7) in thedigital data processor 106 may be implemented by a digital adder 144A.It is noted that digital luminance data may be represented by any numberof bits, depending on the driver IC used (for example, 6-bit, 8-bit,10-bit, 14-bit, etc.).

In FIGS. 3, 4, 6, 8, and 9, the TFT-to-pixel circuit conversionalgorithm module 134 has the measured TFT degradation data 132 or 132Aas an input, and the calculated pixel circuit degradation data 136 as anoutput. However, there may be other inputs to the system to calculatecompensation data as well, as shown in FIG. 10. FIG. 10 illustrates anexample of inputs to the TFT-to-pixel circuit conversion algorithmmodule 134. In FIG. 10, the TFT-to-pixel circuit conversion algorithmmodule 134 processes the measured data (132 of FIGS. 3, 4, 8, and 9;132A of FIG. 5; 132B of FIGS. 8 and 9) based on additional inputs 190(e.g. temperature, other voltages, etc.), empirical constants 192, orcombinations thereof.

The additional inputs 190 may include measured parameters such as avoltage reading from current-programming pixels and a current readingfrom voltage-programming pixels. These pixels may be different from apixel circuit from which the measured signal is obtained. For example, ameasurement is taken from a “pixel under test” and is used incombination with another measurement from a “reference pixel.” Asdescribed below, in order to determine how to modify luminance data to apixel, data from other pixels in the display may be used. The additionalinputs 190 may include light measurements, such as measurement of anambient light in a room. A discrete device or some kind of teststructure around the periphery of the panel may be used to measure theambient light. The additional inputs may include humidity measurements,temperature readings, mechanical stress readings, other environmentalstress readings, and feedback from test structures on the panel

It may also include empirical parameters 192, such as the brightnessloss in the OLED due to decreasing efficiency (ΔL), the shift in OLEDvoltage over time (ΔVoled), dynamic effects of Vt shift, parametersrelated to TFT performance such as Vt, ΔVt, mobility (μ), inter-pixelnon-uniformity, DC bias voltages in the pixel circuit, changing gain ofcurrent-mirror based pixel circuits, short-term and long-term basedshifts in pixel circuit performance, pixel-circuit operating voltagevariation due to IR-drop and ground bounce.

Referring to FIGS. 8 and 9, the TFT-to-pixel-circuit conversionalgorithm in the module 134 and the compensation algorithm 144 in thedigital data processor 106 work together to convert the measured TFTdegradation data 132 into a luminance correction factor. The luminancecorrection factor has information about how the luminance data for agiven pixel is to be modified, to compensate for the degradation in thepixel.

In FIG. 9, the majority of this conversion is done by theTFT-to-pixel-circuit conversion algorithm module 134. It calculates theluminance correction values entirely, and the digital adder 144A in thedigital data processor 106 simply adds the luminance correction valuesto the digital luminance data 104. However, the system 100 may beimplemented such that the TFT-to-pixel circuit conversion algorithmmodule 134 calculates only the degradation values, and the digital dataprocessor 106 calculates the luminance correction factor from that data.The TFT-to-pixel circuit conversion algorithm 134 may employ fuzzylogic, neural networks, or other algorithm structures to convert thedegradation data into the luminance correction factor.

The value of the luminance correction factor may allow the visible lightto remain constant, regardless of the degradation in the pixel circuit.The value of the luminance correction factor may allow the luminance ofdegraded pixels not to be altered at all; instead, the luminance of thenon-degraded pixels to be decreased. In this case, the entire displaymay gradually lose luminance over time, however the uniformity may behigh.

The calculation of a luminance correction factor may be implemented inaccordance with a compensation of non-uniformity algorithm, such as aconstant brightness algorithm, a decreasing brightness algorithm, orcombinations thereof. The constant brightness algorithm and thedecreasing brightness algorithm may be implemented on the TFT-to-pixelcircuit conversion algorithm module (e.g. 134 of FIG. 3) or the digitaldata processor (e.g. 106 of FIG. 3). The constant brightness algorithmis provided for increasing brightness of degraded pixels so as to matchnondegraded pixels. The decreasing brightness algorithm is provided fordecreasing brightness of non-degraded pixels 244 so as to match degradedpixels. These algorithm may be implemented by the TFT-to-pixel circuitconversion algorithm module, the digital data processor (such as 144 ofFIG. 8), or combinations thereof. It is noted that these algorithms areexamples only, and the compensation of non-uniformity algorithm is notlimited to these algorithms.

Referring to FIGS. 11A-11E, the experimental results of the compensationof non-uniformity algorithms are described in detail. Under theexperiment, an AMOLED display includes a plurality of pixel circuits,and is driven by a system as shown in FIGS. 3, 4, 6, 8 and 9. It isnoted that the circuitry to drive the AMOLED display is not shown inFIGS. 11A-11E.

FIG. 11A schematically illustrates an AMOLED display 240 which startsoperating (operation period t=0 hour). The video source (102 of FIGS. 3,4, 7, 8 and 9) initially outputs maximum luminance data to each pixel.No pixels are degraded since the display 240 is new. The result is thatall pixels output equal luminance and thus all pixels show uniformluminance.

Next, the video source outputs maximum luminance data to some pixels inthe middle of the display as shown in FIG. 11B. FIG. 11B schematicallyillustrates the AMOLED display 240 which has operated for a certainperiod where maximum luminance data is applied to pixels in the middleof the display. The video source outputs maximum luminance data topixels 242, while it outputs minimum luminance data (e.g. zero luminancedata) to pixels 244 around the outside of the pixels 242. It maintainsthis for a long period of time, for example 1000 hours. The result isthat the pixels 242 at maximum luminance will have degraded, and thepixels 244 at zero luminance will have no degradation.

At 1000 hours, the video source outputs maximum luminance data to allpixels. The results are different depending on the compensationalgorithm used, as shown in FIGS. 11C-11E.

FIGS. 11C schematically illustrates the AMOLED display 240 to which nocompensation algorithm is applied. As shown in FIG. 11C, if there was nocompensation algorithm, the degraded pixels 242 would have a lowerbrightness than the nondegraded pixels 244.

FIG. 11D schematically illustrates the AMOLED display 240 to which theconstant brightness algorithm is applied. The constant brightnessalgorithm is implemented for increasing luminance data to degradedpixels, such that the luminance data of the degraded pixels matches thatof non-degraded pixels. For example, the increasing brightness algorithmprovides increasing currents to the stressed pixels 242, and constantcurrent to the unstressed pixels 244. Both degraded and nondegradedpixels have the same brightness. Thus, the display 240 is uniform.Differential aging is compensated, and brightness is maintained, howevermore current is required. Since the current to some pixels is beingincreased, this will cause the display to consume more current overtime, and therefore more power over time because power consumption isrelated to the current consumption.

FIG. 11E schematically illustrates the AMOLED display 240 to which thedecreasing brightness algorithm is applied. The decreasing brightnessalgorithm decreases luminance data to nondegraded pixels, such that theluminance data of the nondegraded pixels match that of degraded pixels.For example, the decreasing brightness algorithm provides constant OLEDcurrent to the stressed pixels 242, while decreasing current to theunstressed pixels 244. Both degraded and non-degraded pixels have thesame brightness. Thus, the display 240 is uniform. Differential aging iscompensated, and it requires a lower Vsupply, however brightnessdecrease over time. Because this algorithm does not increase the currentto any of the pixels, it will not result in increased power consumption.

Referring to FIG. 3, components, such as the video source 102 and thedata driver IC 110, may use only 8-bits, or 256 discrete luminancevalues. Therefore if the video source 102 outputs maximum brightness (aluminance value of 255), there is no way to add any additionalluminance, since the pixel is already at the maximum brightnesssupported by the components in the system. Likewise, if the video source102 outputs minimum brightness (a luminance value of 0), there is no wayto subtract any luminance. The digital data processor 106 may implementa grayscale compression algorithm to reserve some grayscales. FIG. 12illustrates an implementation of the digital data processor 106 whichincludes a grayscale compression algorithm module 250. The grayscalecompression algorithm 250 takes the video signal represented by 256luminance values, and transforms it to use less luminance values. Forexample, instead of minimum brightness represented by grayscale 0,minimum brightness may be represented by grayscale 50. Likewise, maximumbrightness may be represented by grayscale 200. In this way, there aresome grayscales reserved for future increase and decrease. It is notedthat the shift in grayscales does not reflect the actual expected shiftin grayscales.

According to the embodiments of the present invention, the scheme ofestimating (predicting) the degradation of the entire pixel circuit andgenerating a luminance correction factor ensures uniformities in thedisplay. According to embodiments of the present invention, the aging ofsome components or entire circuit can be compensated, thereby ensuringuniformity of the display.

According to the embodiments of the present invention, the TFT-to-pixelcircuit conversion algorithm allows for improved display parameters, forexample, including constant brightness uniformity and color uniformityacross the panel over time. Since the TFT-to-pixel circuit conversionalgorithm takes in additional parameters, for example, temperature andambient light, any changes in the display due to these additionalparameters may be compensated for.

The TFT-to-Pixel circuit conversion algorithm module (134 of FIGS. 3, 4,6, 8 and 9), the compensation module (144 of FIG. 8, 144A of FIG. 9, thecompensation of non-uniformity algorithm, the constant brightnessalgorithm, the decreasing brightness algorithm and the grayscalecompression algorithm may be implemented by any hardware, software or acombination of hardware and software having the above describedfunctions. The software code, instructions and/or statements, either inits entirety or a part thereof, may be stored in a computer readablememory. Further, a computer data signal representing the software code,instructions and/or statements, which may be embedded in a carrier wavemay be transmitted via a communication network. Such a computer readablememory and a computer data signal and/or its carrier are also within thescope of the present invention, as well as the hardware, software andthe combination thereof.

Referring again to FIG. 3, which illustrates the operation of the lightemitting display system 100 by applying a compensation algorithm todigital data 104. In particular, FIG. 3 illustrates the operation of apixel in an active matrix organic light emitting diode (AMOLED) display.The display system 100 includes an array of pixels. The video source 102includes luminance input data for the pixels. The luminance data is sentin the form of digital input data 104 to the digital data processor 106.The digital input data 104 can be eight-bit data represented as integervalues existing between 0 and 255, with greater integer valuescorresponding to higher luminance levels. The digital data processor 106can optionally manipulate the digital input data 104 by, for example,scaling the resolution of the video source 102 to a native screenresolution, adjusting the color balance, or applying a gamma correctionto the video source 102. The digital data processor 106 can also applydegradation corrections to the digital input data 104 based ondegradation data 136. Following the manipulations, the digital dataprocessor 106 sends the resulting digital data 108 to the data driverintegrated circuit (IC) 110. The data driver IC 110 converts the digitaldata 108 into the analog voltage or current output 112. The data driverIC 110 can be implemented, for example, as a module including a digitalto analog converter. The analog voltage or current 112 is provided tothe pixel circuit 114. The pixel circuit 114 can include an organiclight emitting diode (OLED) and thin film transistors (TFTs). One of theTFTs in the pixel circuit 114 can be a drive TFT that applies a drivecurrent to the OLED. The OLED emits visible light 126 responsive to thedrive current flowing to the OLED. The visible light 126 is emitted witha luminance related to the amount of current flowing to the OLED throughthe drive TFT.

In a configuration where the analog voltage or current 112 is aprogramming voltage, the drive TFT within the pixel circuit 114 cansupply the OLED according to the analog voltage or current 112 by, forexample, biasing the gate of the drive TFT with the programming voltage.The pixel circuit 114 can also operate where the analog voltage orcurrent 112 is a programming current applied to each pixel rather than aprogramming voltage. A display system 100 utilizing programming currentscan use current minors in each pixel circuit 114 to apply a drivecurrent to the OLED through the drive TFT according to the programmingcurrent applied to each pixel.

The luminance of the emitted visible light 126 is affected by aspectswithin the pixel circuit 114 including the gradual degradation ofhardware within the pixel circuit 114. The drive TFT has a thresholdvoltage, and the threshold voltage can change over time due to aging andstressing of the drive TFT. The luminance of the emitted visible light126 can be influenced by the threshold voltage of the drive TFT, thevoltage drop across the OLED, and the efficiency of the OLED. Theefficiency of the OLED is a ratio of the luminance of the emittedvisible light 126 to the drive current flowing through the OLED.Furthermore, the degradation can generally be non-uniform across thedisplay system 100 due to, for example, manufacturing tolerances of thedrive TFTs and OLEDs and differential aging of pixels in the displaysystem 100. Non-uniformities in the display 100 are generally referredto as display mura or defects. In a display 100 with an array of OLEDshaving uniform light emitting efficiency and threshold voltages drivenby TFTs having uniform gate threshold voltages, the luminance of thedisplay will be uniform when all the pixels in the display areprogrammed with the same analog voltage or current 112. However, as theOLEDs and TFTs in each pixel age and the degradation characteristicschange, the luminance of the display ceases to be uniform whenprogrammed the same.

The degradation can be compensated for by increasing the amount of drivecurrent sent through the OLED in the pixel circuit 114. According to animplementation of the present disclosure, compensation for thedegradation of the display 100 can be carried out by adjusting thedigital data 108 output from the digital data processor 106. The digitaldata processor 106 receives the degradation data 136 from thecompensation module 130. The compensation module 130 receivesdegradation data 132 based on measurements of parameters within thepixel circuit 114. Alternatively, the degradation data 132 sent to thecompensation module 130 can be based on estimates of expectedperformance of the hardware aspects within the pixel circuit 114. Thecompensation module 130 includes the module 134 for implementing thealgorithm 134, such as the TFT-to-pixel circuit conversion algorithm.The degradation data 132 can be electrical data that represents how mucha hardware aspect of the pixel circuit 114 has been degraded. Thedegradation data 132 measured or estimated from the pixel circuit 114can represent one or more characteristics of the pixel circuit 114.

In a configuration where the analog voltage or current 112 is aprogramming voltage, the programming voltage is generally determined bythe digital input data 104, which is converted to a voltage in the datadriver IC 110. The present disclosure provides a method of compensatingfor non-uniform characteristics in each pixel circuit 114 that affectthe luminance of the emitted visible light 126 from each pixel.Compensation is performed by adjusting the digital input data 104 in thedigital data processor 106 before the digital data 108 is passed to thedata driver IC 110.

FIG. 13 is a data flow chart showing the compression and compensation ofluminosity input data 304 used to drive an AMOLED display. The data flowchart shown in FIG. 13 includes a digital data processor block 306 thatcan be considered an implementation of the digital data processor 106shown in FIG. 3. Referring again to FIG. 13, a video source provides theluminosity input data 304. The input data 304 is a set of eight-bitinteger values. The input data 304 includes integer values that existbetween 0 and 255, with the values representing 256 possibleprogrammable luminosity values of the pixels in the AMOLED display. Forexample, 255 can correspond to a pixel programmed with maximumluminance, and 127 can correspond to a pixel programmed with roughlyhalf the maximum luminance. The input data 304 is similar to the digitalinput data 104 shown in FIG. 3. Referring again to FIG. 13, the inputdata 304 is sent to the digital data processor block 304. In the digitaldata processor block 304, the input data 304 is multiplied by four (310)in order to translate the eight-bit input data 304 to ten-bit resultingdata 312. Following the multiplication by four (310), the resulting data312 is a set of ten-bit integers existing between 0 and 1020.

By translating the eight-bit input data 304 to the ten-bit resultingdata 312, the resulting data 312 can be manipulated for compensation ofluminance degradation with finer steps than can be applied to theeight-bit input data 304. The ten-bit resulting data 312 can also bemore accurately translated to programming voltages according to a gammacorrection. The gamma correction is a non-linear, power law correctionas is appreciated in the art of display technology. Applying the gammacorrection to the input data can be advantageous, for example, toaccount for the logarithmic nature of the perception of luminosity inthe human eye. According to an aspect of the present disclosure,multiplying the input data 304 by four (310) translates the input data304 into a higher quantized domain. While the present disclosureincludes multiplying by four (310), in an implementation the input data304 can be multiplied by any number to translate the input data 310 intoa higher quantized domain. The translation can advantageously utilizemultiplication by a power of two, such as four, but the presentdisclosure is not so limited. Additionally, the present disclosure canbe implemented without translating the input data 304 to a higherquantized domain.

The resulting data 312 is multiplied by a compression factor, K (314).The compression factor, K, is a number with a value less than one.Multiplying the resulting data 312 by K (314) allows for scaling theten-bit resulting data 312 into compressed data 316. The compressed data316 is a set of ten-bit integers having values ranging from 0 to theproduct of K and 1020. Next, the compressed data 316 is compensated fordegradations in the display hardware (318). The compressed data 316 iscompensated by adding additional data increments to the integerscorresponding to the luminance of each pixel (318). The compensation fordegradation is performed according to degradation data 336 that is sentto the digital data processor block 306. The degradation data 336 isdigital data representing an amount of compensation to be applied to thecompressed data 316 within the digital data processor block 306according to degradations in the display hardware corresponding to eachpixel. Following the compensation for degradations (318), compensateddata 308 is output. The compensated data 208 is a set of ten-bit integervalues with possible values between 0 and 1023. The compensated data 308is similar in some respects to the digital data 108 output from thedigital data processor 106 in FIG. 3. Referring again to FIG. 13, thecompensated data 308 is supplied to a display driver, such as a displaydriver incorporating a digital to analog converter, to createprogramming voltages for pixels in the AMOLED display.

The degradations in the display hardware can be from mura defects(non-uniformities), from the OLED voltage drop, from the voltagethreshold of the drive TFT, and from changes in the OLED light emittingefficiency. The degradations in the display hardware each generallycorrespond to an additional increment of voltage that is applied to thepixel circuit in order to compensate for the degradations. For aparticular pixel, the increments of additional voltage necessary tocompensate for the hardware degradations can be referred to as:V_(mura), V_(Th), V_(OLED), and V_(efficiency). Each of the hardwaredegradations can be mapped to corresponding increments in data stepsaccording to a function of V_(mura), V_(Th), V_(OLED), V_(efficiency),D(V_(mura), V_(Th), V_(OLED), V_(efficiency)). For example, therelationship can be given by Expression 1: D(V_(mura), V_(Th), V_(OLED),V_(efficiency))=int[(2^(nBits)−1)(V_(mura)+V_(Th)+V_(OLED)+V_(efficiency))/V_(Max)], where nBits is thenumber of bits in the data set being compensated and V_(Max) is themaximum programming voltage. In Expression 1, int[ ] is a function thatevaluates the contents of the brackets and returns the nearest integer.The degradation data 336 sent to the digital data processor block 306can be digital data created according to the relationship forD(V_(mura), V_(Th), V_(OLED), V_(efficiency)) provided in Expression 1.In an implementation of the present disclosure, the degradation data 336can be an array of digital data corresponding to an amount ofcompensation to be applied to the compressed data of each pixel in anAMOLED display. The array of digital data is a set of offset incrementsthat can be applied to the compressed data by adding the offsetincrements to the compressed data of each pixel or by subtracting theoffset increments from the compressed data of each pixel. The set ofoffset increments can generally be a set of digital data with entriescorresponding to an amount of compensation needed to be applied to eachpixel in the AMOLED display. The amount of compensation can be theamount of increments in data steps needed to compensate for adegradation according to Expression 1. In a configuration, locations inthe array of the degradation data 336 can correspond to locations ofpixels in the AMOLED display.

For example, Table 1 below provides a numerical example of thecompression of input data according to FIG. 13. Table 1 provides examplevalues for a set of input data 304 following the multiplication by four(310) and the multiplication by K (314). In the example provided inTable 1, K has a value of 0.75. In Table 1, the first column providesexample values of integer numbers in the set of input data 304. Thesecond column provides example values of integer numbers in the set ofresulting data 312 created by multiplying the corresponding input datavalues by four (310). The third column provides example values ofnumbers in the set of compressed data 316 created by multiplying thecorresponding values of the resulting data 312 by K, where K has anexample value of 0.75. The final column is the output voltagecorresponding to the example compressed data 316 shown in the thirdcolumn when no compensation is applied. The final column is created foran example display system having a maximum programming voltage of 18 V.In the numerical example illustrated in Table 1, the programming outputvoltage corresponding to the input data with the maximum input oftwo-hundred fifty-five is more than 4.5 V below the maximum voltage. The4.5 V can be considered the compensation budget of the display system,and can be referred to as the voltage headroom, V_(headroom). Accordingto an aspect of the present disclosure, the 4.5 V is used to providecompensation for degradation of pixels in the AMOLED display.

TABLE 1 Numerical Example of Input Data Compression Output VoltageResulting Data Compressed Data (without degradation Input Data (×4)(×0.75) compensation) 255 1020 765 13.46 V 254 1016 762 13.40 V 253 1012759 13.35 V . . . . . . . . . . . . 2 8 6  0.10 V 1 4 3  0.05 V 0 0 0 0.00 V

According to an implementation of the present disclosure, the amount ofvoltage available for providing compensation degradation isV_(headroom). An amount of V_(headroom) can be advantageously reservedto compensate for a degradation of a pixel in an AMOLED display with themost severe luminance degradation. By reserving an amount ofV_(headroom) to compensate for the most severely degraded pixel, therelative luminosity of the display can be advantageously maintained. Therequired amount of V_(headroom) to compensate for the pixel in an AMOLEDdisplay with a maximum amount of degradation is given by Expression 2:V_(headroom)=max[V_(mura)+V_(Th)+V_(OLED)+V_(efficiency)]. In Expression2, V_(mura), V_(Th), V_(OLED), and V_(efficiency) can be an array ofvalues corresponding to the amount of additional voltage necessary tocompensate the pixels in the display, and the entries in the arrays ofvalues can correspond to individual pixels in the display. That is,V_(mura) can be an array of voltages required to compensate display muraor non-uniform defects; V_(Th) can be an array of voltage thresholds ofdrive TFTs of pixels in the display; V_(OLED) can be an array of OLEDvoltages of the pixels in the display; and V_(efficiency) can be anarray of voltages required to compensate for OLED efficiencydegradations of pixels in the display. In Expression 2, max[ ] is afunction evaluating an array of values in the brackets and returning themaximum value in the array.

As can be appreciated with reference to FIG. 13 and Table 1, the choiceof K affects the amount of V_(headroom) available to compensate fordegradations in the display. Choosing a lower value of K leads to agreater amount of V_(headroom). In a configuration of the presentdisclosure where the need for compensation increases over time due toaging of the display, the value of K can be advantageously decreasedover time according to the degradation of the display over time.Decreasing K enables uniformity compensation across the display suchthat pixels receiving the same digital input data actually emit lightwith the same luminance, but the uniformity compensation comes at thecost of overall luminance reduction for the entire display. FIGS. 14through 17 provide methods for selecting and adjusting K.

FIG. 14 is a flowchart illustrating a method for selecting thecompression factor according to display requirements and the design ofthe pixel circuit. In operation of the method illustrated by theflowchart in FIG. 14, the display requirements and pixel circuit designof a display are analyzed to estimate maximum values of V_(mura),V_(Th), V_(OLED), and V_(efficiency) for the pixels in the display(405). The estimation (405) can be carried out based on, for example,empirical data from experimental results related to the aging ofdisplays incorporating pixel circuits similar to the pixel circuit inthe display 100. Alternatively, the estimation (405) can be carried outbased on numerical models or software-based simulation models ofanticipated performances of the pixel circuit in the display 100. Theestimation (405) can also account for an additional safety margin ofheadroom voltage to account for statistically predictable variationsamongst the pixel circuits in the display 100. Responsive to theestimation (405), the required voltage headroom is calculated (410). Therequired voltage headroom, V_(headroom), is calculated according toExpression 2. Once V_(headroom) is calculated, the compression factor,K, is calculated (415) according to Expression 3: K=1−V_(headroom)/V_(Max), where V_(Max) is a maximum programming voltage forthe display 100. The compression factor, K, is then set (420) for use inthe compression and compensation algorithm, such as the compressionalgorithm illustrated in the data flow chart in FIG. 13.

FIG. 15 is a flowchart illustrating a method for selecting thecompression factor according to a pre-determined headroom adjustmentprofile. A headroom adjustment profile is selected (505). The firstblock 505 in the flowchart in FIG. 15 graphically illustrates threepossible headroom adjustment profiles as profile 1, profile 2, andprofile 3. The profiles illustrated are graphs of K versus time. Thetime axis can be, for example, a number of hours of usage of the display100. In all three profiles K decreases over time. By decreasing K overtime, an additional amount of voltage (V_(headroom)) is available forcompensation. The example profiles in the first block 505 includeprofile 1, which maintains K at a constant level until a time thresholdis reached and K decreases linearly with usage time thereafter. Profile2 is a stair step profile, which maintains K at a constant level for atime, and then decreases K to a lower value, when it is maintained untilanother time, at which point it is decreased again. Profile 3 is alinear decrease profile, which provides for K to gradually decreaselinearly with usage time. The profile can be selected by a user profilesetting according to a user's preferences for the compensationtechniques employed over the life of the display. For example, a usermay want to maintain an overall maximum luminance for the display for aspecific amount of usage hours before dropping the luminance. Anotheruser may be fine with gradually dropping the luminance from thebeginning of the display's lifetime.

Once an headroom adjustment profile is selected (505), the display usagetime is monitored (510). At a given usage time, the value of thecompression factor, K, is determined according to the usage time andselected profile (515). The compression factor, K, is then set (520),and the display usage time continues to be monitored (510). After K isset (520), K can be used in the compression and compensation algorithm,such as the compression algorithm illustrated in the data flow chart inFIG. 13. According to an aspect of the present disclosure, the method ofsetting and adjusting K shown in FIG. 15 is a dynamic method of settingand adjusting K, because the value of K is updated over time accordingto the usage time of the display 100.

FIG. 16 is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a threshold over a previous compensation. Measurements aretaken from aspects of the pixel circuits of the pixels in the display100 to measure V_(mura), V_(Th), V_(OLED), and V_(efficiency) (605) andcompute V_(headroom) according to Expression 2. The difference betweenthe value of V_(headroom) presently computed at time t2 is then comparedto the value of V_(headroom) computed at an earlier time t1 by computingthe difference (610). The difference is ΔV_(headroom), and is calculatedaccording to Expression 5:ΔV_(headroom)=(V_(headroom))_(t2)−(V_(headroom))_(t1). In Expression 5,t1 is the last time used to adjust the compensation factor, K, and t2 isa present time. The subscripts in the right hand side of Expression 5indicate a time of evaluation of the quantity in parentheses.

The calculated value of ΔV_(headroom) is then compared to a compensationthreshold, V_(thresh) (615). If ΔV_(headroom) exceeds V_(thresh), K ismodified (620). If ΔV_(headroom) is less than or equal to V_(thresh), Kis not modified. The value of K can be modified according to Expression6: K_(new)=K_(old)/A−B, where K_(new) is the new value of K, K_(old) isthe old value of K, and A and B are values set for applications anddifferent technologies. For example, A and B can be set based onempirical results from experiments examining the characteristicdegradation due to aging of pixel circuits similar to those used in thedisplay 100 to drive OLEDs in each pixel. Similar measurements or userinputs can be used to set V_(thresh) as well. The compression factor, K,is then set (625) for use in the compression and compensation algorithm,such as the compression algorithm illustrated in the data flow chart inFIG. 13. Degradation measurements continue to be measured (605),ΔV_(headroom) continues to be calculated (610), and K is updatedaccording to Expression 6 whenever ΔV_(headroom) exceeds V_(thresh)(515). According to an aspect of the present disclosure, the method ofadjusting K shown in FIG. 5 is a dynamic method of adjusting K, becausethe value of K is updated over time according to degradationmeasurements gathered from the pixel circuits within the display 100.

Alternatively, the compression factor can be modified (620) according toExpression 3 based on the measured V_(headroom). According to an aspectof the method provided in the flowchart shown in FIG. 16, the value of Kis maintained until a threshold event occurs (615), when K is modified(620). Implementing the method provided in FIG. 16 for adjusting thecompression factor, K, can result in K being decreased over timeaccording to a stair step profile.

FIG. 17 is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a previously measured maximum. Measurements are taken fromaspects of the pixel circuits of the pixels in the display 100 tomeasure V_(mura), V_(Th), V_(OLED), and V_(efficiency) (605). Themeasurements of V_(mura), V_(Th), V_(OLED), and V_(efficiency) arereferred to as degradation measurements. The maximum values of thedegradation measurements are selected (710). The maximum values of thedegradation can be selected according to Expression 2. The combinationof measuring the degradation measurements (605) and selecting themaximum values (710) provides for ascertaining the maximum compensationapplied to pixels within the display. The maximum values are compared topreviously measured maximum values of previously measured degradationmeasurements (715). If the presently measured maximum values exceed thepreviously measured maximum values, V_(headroom) is calculated accordingto Expression 2 (410) based on the present degradation measurements.Next, the compression factor, K, is determined according to Expression 3(720). The compression factor is set (725) and the maximum values areupdated for comparison with new maximum values (715). The compressionfactor is set (725) for use in the compression and compensationalgorithm, such as the compression algorithm illustrated in the dataflow chart in FIG. 13. Similar to the method provided in FIG. 16, themethod shown illustrated by the flowchart in FIG. 17 is a dynamic methodof adjusting K based on degradation measurements continually gatheredfrom the pixel circuits within the display 100.

The present disclosure can be implemented by combining the abovedisclosed methods for setting and adjusting the compression factor, K,in order to create an adequate amount of voltage headroom that allowsfor compensation to be applied to the digital data before it is passedto the data driver IC. For example, a method of setting and adjusting Kaccording to FIG. 16 or FIG. 17 can also incorporate a user selectedprofile as in FIG. 15.

In an implementation of the present disclosure, the methods of selectingand adjusting the compression factor, K, provided in FIGS. 14 through 17can be used in conjunction with the digital data manipulationsillustrated in FIG. 13 to operate a display while maintaining theuniform luminosity of the display. In a configuration, the abovedescribed methods allow for maintaining the relative luminosity of adisplay by compensating for degradations to pixels within the display.In a configuration, the above described methods allow for maintainingthe luminosity of a pixel in a display array for a given digital inputby compensating for degradations within the pixel's pixel circuit.

The present disclosure describes maintaining uniform luminosity of anAMOLED display, but the techniques presented are not so limited. Thedisclosure is applicable to a range of systems incorporating arrays ofdevices having a characteristic stimulated responsive to a data input,and where the characteristic is sought to be maintained uniformly. Forexample, the present disclosure applies to sensor arrays, memory cells,and solid state light emitting diode displays. The present disclosureprovides for modifying the data input that stimulates the characteristicof interest in order to maintain uniformity. While the presentdisclosure for compressing and compensating digital luminosity data tomaintain a luminosity of an AMOLED display is described as utilizingTFTs and OLEDs, the present disclosure applies to a similar apparatushaving a display including an array of light emitting devices.

While particular embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise construction and compositionsdisclosed herein and that various modifications, changes, and variationscan be apparent from the foregoing descriptions without departing fromthe spirit and scope of the invention as defined in the appended claims.

1. A method of compensating for a degradation of a pixel having adriving circuit for driving current through a light emitting devicebased on an input, the method comprising: receiving luminosity data;scaling the luminosity data by a compression factor to create compresseddata; compensating for the degradation of the pixel by adjusting thecompressed data to create compensated data; and supplying the drivingcircuit based on the compensated data.
 2. The method of claim 1, whereinthe scaling is carried out by multiplying the luminosity data by aconstant integer to create resulting data with a greater number of bits,and multiplying the resulting data by the compression factor.
 3. Themethod of claim 2, wherein the luminosity data is an eight-bit integerand the compressed data is a ten-bit integer.
 4. The method of claim 1,wherein the driving circuit includes at least one thin film transistor(TFT).
 5. The method of claim 4, wherein the at least one TFT is ann-type TFT.
 6. The method of claim 4, wherein the at least one TFT isused to drive current through the light emitting device, and wherein thedegradation is due to a voltage threshold of the at least one TFT or dueto a shift in the voltage threshold of the at least one TFT.
 7. Themethod of claim 1, wherein the light emitting device is an organic lightemitting diode (OLED).
 8. The method of claim 7, wherein the degradationis due to a bias voltage of the OLED or due to a shift in the biasvoltage of the OLED.
 9. The method of claim 7, wherein the degradationis due to a voltage required to compensate for an inefficiency of theOLED or due to a shift in the voltage required to compensate for theinefficiency of the OLED.
 10. The method of claim 1, wherein thecompression factor is determined based on a user selected profile and ausage time of the pixel.
 11. The method of claim 1, wherein thecompression factor is determined based on an estimation of degradationof the pixel and on a display requirement, and wherein the estimation isbased on a design of hardware aspects of the pixel and of the drivingcircuit.
 12. A method of operating a display having a plurality ofpixels to compensate for a degradation of a pixel in the display, thepixel having a driving circuit for driving a current through a lightemitting device based on an input, the input being supplied to thedriving circuit by a display driver, the method comprising: receivingluminosity data; scaling the luminosity data by a compression factor tocreate compressed data; compensating for a degradation of a pixel in thedisplay by adjusting the compressed data based on the degradation tocreate compensated data; and sending the compensated data to the displaydriver.
 13. The method of claim 12, further comprising: ascertaining amaximum compensation applied to the plurality of pixels; and adjustingthe compression factor based on the ascertained maximum compensation.14. The method of claim 12, further comprising: compensating fordegradations of the plurality of pixels in the display by adjusting thecompressed data based on the degradations to create compensated data;14. The method of claim 13, wherein the adjusting is carried out bycomputing the ratio of the ascertained maximum compensation to a maximumassignable value of the inputs and updating the compression factor to beone minus the computed ratio.
 15. The method of claim 12, wherein theluminosity data includes eight-bit integers and wherein the scaling iscarried out by: multiplying the luminosity data by a constant integer tocreate resulting data with a greater number of bits, and multiplying theresulting data by the compression factor.
 16. The method of claim 12,wherein at least one of the driving circuits includes at least one thinfilm transistor (TFT).
 17. The method of claim 16, wherein the at leastone TFT is an n-type TFT.
 18. The method of claim 16, wherein the atleast one TFT is used to drive current through at least one of the lightemitting devices, and wherein the degradation is due to a voltagethreshold of the at least one TFT or due to a shift in the voltagethreshold of the at least one TFT.
 19. The method of claim 12, whereinat least one of the light emitting devices is an organic light emittingdiode (OLED).
 20. The method of claim 19, wherein the degradation is dueto a bias voltage of the OLED or due to a shift in the bias voltage ofthe OLED.
 21. The method of claim 19, wherein the degradation is due toa voltage required to compensate for an inefficiency of the OLED or dueto a shift in the voltage required to compensate for the inefficiency ofthe OLED.
 22. The method of claim 12, wherein the compression factor isdetermined based on a user selected profile and a usage time of thedisplay.
 23. The method of claim 12, wherein the compression factor isdetermined based on an estimation of the degradation of the display andbased on display requirements and the design of hardware aspects withinthe display.
 24. A method of operating a display having a plurality ofpixels to compensate for degradation of the plurality of pixels, whereinthe plurality of pixels have driving circuits for driving currentsthrough light emitting devices based on inputs, the method comprising:operating the display according to a first compression factor by:receiving a first set of luminosity data for the plurality of pixels;scaling the first set of luminosity data by the first compression factorto create a first set of compressed data; compensating for a firstdegradation of the plurality of pixels by adjusting the first set ofcompressed data based on a first set of offset increments to create afirst set of compensated data; and supplying the driving circuits basedon the first set of compensated data; determining a second compressionfactor based on a second degradation of the plurality of pixels; andoperating the display according to the second compression factor by:receiving a second set of luminosity data for the plurality of pixels;scaling the second set of luminosity data by the second compressionfactor to create a second set of compressed data; compensating for thesecond degradation of the plurality of pixels by adjusting the secondset of compressed data based on a second set of offset increments tocreate a second set of compensated data; and supplying the drivingcircuits based on the second set of compensated data.
 25. The method ofclaim 24, further comprising: prior to operating the display accordingto the first compression factor, determining the first compressionfactor based on the first degradation of the plurality of pixels. 26.The method of claim 24, wherein the adjusting the first set ofcompressed data is carried out by adding the first set of offsetincrements to the first set of compressed data to create the first setof compensated data, and wherein the adjusting the second set ofcompressed data is carried out by adding the second set of offsetincrements to the second set of compressed data to create the second setof compensated data.
 27. The method of claim 24, wherein the adjustingthe first set of compressed data is carried out by subtracting the firstset of offset increments from the first set of compressed data to createthe first set of compensated data, and wherein the adjusting the secondset of compressed data is carried out by subtracting the second set ofoffset increments from the second set of compressed data to create thesecond set of compensated data.
 28. The method of claim 25, wherein thedetermining the first compression factor is carried out by ascertainingthe maximum value in the first set of offset increments and computingthe ratio of the ascertained maximum to a maximum assignable inputvalue, and wherein the first set of offset increments is determinedbased on estimates of degradation of the plurality of pixels.
 29. Themethod of claim 25, wherein the determining the first compression factoris carried out by ascertaining the maximum value in the first set ofoffset increments and computing the ratio of the ascertained maximum toa maximum assignable input value, and wherein the first set of offsetincrements is determined based on measurements of degradation of theplurality of pixels.
 30. The method of claim 24, wherein the determiningthe second compression factor is carried out by ascertaining the maximumvalue in the second set of offset increments and computing the ratio ofthe ascertained maximum to a maximum assignable input value, and whereinthe second set of offset increments is determined based on estimates ofdegradation of the plurality of pixels.
 31. The method of claim 24,wherein the determining the second compression factor is carried out byascertaining the maximum value in the second set of offset incrementsand computing the ratio of the ascertained maximum to a maximumassignable input value, and wherein the second set of offset incrementsis determined based on measurements of degradation of the plurality ofpixels.
 32. The method of claim 24, wherein the first set of luminositydata and second set of luminosity data include eight-bit integers, andwherein the scaling the first set of luminosity data is carried out by:multiplying the first set of luminosity data by a constant integer tocreate a first set of resulting data that includes integers having anumber of bits greater eight; and multiplying the first set of resultingdata by the first compression factor, and wherein the scaling the secondset of luminosity data is carried out by: multiplying the second set ofluminosity data by the constant integer to create a second set ofresulting data that includes integers having a number of bits greatereight; and multiplying the second set of resulting data by the secondcompression factor.
 33. A display degradation compensation system forcompensating for a degradation of a plurality of pixels in a display,wherein the plurality of pixels have driving circuits for drivingcurrents through light emitting devices, the display degradationcompensation system comprising: a digital data processor module forreceiving a luminosity data, compressing the luminosity data accordingto a compression factor, and compensating for the degradation of theplurality of pixels by adjusting the compressed data to createcompensated data; and a display driver for receiving the compensateddata and supplying the inputs to the driving circuits, the drivingcircuits being configured to deliver the driving currents to the lightemitting devices based on the received compensated data.
 34. The displaydegradation compensation system of claim 33, wherein the adjusting thecompressed data is carried out according to a measurement of thedegradation of the plurality of pixels.
 35. The display degradationcompensation system of claim 33, wherein the digital data processormodule includes a digital adder for adjusting the compressed data tocreate compensated data.
 36. The display degradation compensation systemof claim 33, further comprising: a compensation module for determiningthe compression factor.
 37. The display degradation compensation systemof claim 36, wherein the compensation module is configured to determinethe compression factor according to a function including a measurementof the degradation of the plurality of pixels.
 38. The displaydegradation compensation system of claim 36, wherein the compensationmodule is configured to dynamically adjust the compression factoraccording to an input specified by a user and according to a usage timeof the display.
 39. The display degradation compensation system of claim36, wherein the compensation module is configured to dynamically adjustthe compression factor according to a function including a measurementof the degradation of the plurality of pixels.
 40. The displaydegradation compensation system of claim 33, wherein the digital dataprocessor module is configured to receive eight-bit luminance data andoutput ten-bit compensated data.
 41. The display degradationcompensation system of claim 33, wherein at least one of the lightemitting devices is an organic light emitting diode.
 42. The displaydegradation compensation system of claim 33, wherein at least one of thedriving circuits includes at least one thin film transistor.
 43. Amethod of operating a display having a plurality of pixels to compensatefor a degradation of the plurality of pixels, wherein the plurality ofpixels have driving circuits for driving currents through light emittingdevices based on inputs, the method comprising: receiving a video signalhaving a set of grayscale values for the plurality of pixels;compressing the grayscale values to a range less than an available rangeand reserving a remaining amount of grayscale values for adjustments;and outputting the compressed grayscale values to a display driver forprogramming the driving circuits of the plurality of pixels.
 44. Themethod of claim 43, wherein the set of grayscale values is eight-bitdata, and wherein the compressing is carried out according to agrayscale compression algorithm that translates the set of compressedgrayscale values having a range of 200 values.
 45. The method of claim43, wherein the remaining amount of grayscale values are reserved at ahigh end of the available range to provide adjustments that increase theluminosity of adjusted pixels.
 46. The method of claim 43, wherein theremaining amount of grayscale values are reserved at a low end of theavailable range to provide adjustments that decrease the luminosity ofadjusted pixels.
 47. The method of claim 43, further comprising:adjusting a value in the set of grayscale values corresponding to atleast one of the plurality of pixels according to a decreasingbrightness algorithm.
 48. The method of claim 43, further comprising:adjusting a value in the set of grayscale values corresponding to atleast one of the plurality of pixels according to a constant brightnessalgorithm.
 49. A system for compensating for non-uniformities in adisplay having a plurality of pixels, at least one of the plurality ofpixels including a pixel circuit having a light emitting device, thepixel circuit configured to drive the pixel based on luminance data; thesystem comprising: a module for modifying the pixel data applied to oneor more than one pixel, the module including: an estimating module forestimating a degradation of a first pixel circuit based on measurementdata read from the first pixel circuit; a grayscale compression modulefor compressing the luminance data according to a grayscale compressionalgorithm to reserve grayscale values; and a compensating module forcorrecting the compressed luminance data applied to the first or asecond pixel circuit based on the estimation of the degradation of thefirst pixel circuit; and a display driver for receiving the correctedluminance data and supplying the pixel circuit with an analog voltage orcurrent based on the corrected luminance data.
 50. The system of claim49, wherein the grayscale compression module transforms the luminancedata so as to use luminance values less than those of the originalluminance data.
 51. The system of claim 49, wherein the luminance datais eight-bit data, and wherein the compressing is carried out in thegrayscale compression module to transform the luminance data to a rangeof 200 values.
 52. The method of claim 49, wherein the reservedgrayscale values are reserved at a high end of an available range toallow for providing corrections to the compressed luminance data thatincrease the luminosity of corrected pixels.
 53. The method of claim 49,wherein the reserved grayscale values are reserved at a low end of anavailable range to allow for providing corrections to the compressedluminance data that decrease the luminosity of corrected pixels.
 54. Themethod of claim 49, wherein the compensating module corrects theluminance data according to a decreasing brightness algorithm.
 55. Themethod of claim 49, wherein the compensating module corrects theluminance data according to a constant brightness algorithm.